The present invention relates to a method for producing coal pellets and, more particularly, a method for producing coal pellets to be used as fuel, which can be pneumatically transported and burnt in a coal burner of a boiler or the like.
Conventionally, fuel coal pellets have been prepared by adding a binder comprising a heavy oil, tar or pitch (hereinafter simply referred to as heavy oil or oil) to pulverized coal particles. Coal pellets so-produced appear to be dry solid particles but, in reality, have relatively high adhesion. When these fuel coal pellets are pulverized in a mill and then supplied by a pneumatic transport to, for example, a coal burner of a boiler, they have a strong tendency to attach and accumulate on the inner wall surface of the mill and the pneumatic transport pipe. As the operation time lapses, they undergo growing accumulation, so as to eventually clog the flow path in the operation machinery, resulting in that the intended burning can no longer continue. This also results in gradual lowering of the pulverization power of the mill, so that the mill eventually cannot produce the prescribed yield of pulverized coal particles.
As a result of extensive analytical studies of the above-discussed difficulties in the art, the present inventors discovered the following:
(1) In conventional coal-pellets production, the oil binder component was used in excessive amounts. In particular, according to conventional production methods, no consideration was given the fact that the product coal pellets are further processed for pulverization, and thereafter, pneumatically transported so as to be burnt in a coal burner. Also, in the conventional production of coal pellets, attention was only paid to such factors as the ease of pelletizing (generally speaking, the use of more oil component resulted in a greater ease of pelletizing). Thus, the above-discussed problem of adhesion of the coal particles was created as a matter of course.
(2) According to conventional methods, the oil component is used in an excessive amount, resulting in the product coal pellets lacking a desirable grindability. Namely, when conventional coal pellets have been pulverized, oil films were formed on the surface of the resultant coal particles, which provides a cushioning function. This adversely affects the transmission of a pulverization load onto the coal pellets, so that pulverization cannot effectively proceed.
In order to avoid the above-discussed difficulties, reduction of the amount of the oil binder used in the production of coal pellets was attempted. To this end, small scale tests were conducted using a relatively small amount of raw coal, and attempts were made to determine a minimum amount of the oil component required for production of coal pellets and, based on the data obtained, determine the preferred use amount of the oil component for an industrial scale of coal pellet production.
As a result of the above, the following facts are uncovered:
That the amount of the oil component required for the pelletizing is variable according to a change in the rank of coalification, ash content and water content in or of the material or raw coal,
That raw coal products of a same place of origin and a same brand have varied properties depending upon a change in the bed depth where they were produced, and accordingly, they require varied amounts of the oil component for their pelletizing,
That in handling an enormous volume of coal over a relatively great length of time, as is normally the case with the coal handling, physical properties of the coal surfaces, such as for example convexo-concave surface conditions, the size and distribution of capillary voids and ash contents of coal can change lot by lot and/or as time lapses, and
That data as obtained above as a result of small scale test are not applicable to operations in an industrial scale plant and that only by way of suppressing the use amount of the oil component, it is feasible to provide a solution to the problems and difficulties discussed in the aforementioned passages (1) and (2).